Fix mi_expect_stop for non-zero exit codes

Fix mi_expect_stop for non-zero exit codes

The message displayed by gdb is different when the inferior exits with
zero and non-zero values, this fix takes that into account.

gdb/testsuite/ChangeLog:
2014-05-13  Simon Marchi  <simon.marchi@ericsson.com>

	* lib/mi-support.exp (mi_expect_stop): Expect message for
	inferiors that exit with non-zero exit code.